What is a Galaxy (& what is the Milky Way)?

Most people are familiar with the words solar system, galaxy, Milky Way and Universe, and most people probably have a vague idea of what they mean. I’ve been throwing these words around quite a bit on the blog but I realized I’ve never actually explained what they are.

I thought I had a pretty good understanding of what they actually mean, but in an earlier post I defined what a solar system is, and I learnt a fair bit myself about where a solar system actually ends. The same goes with this post, I knew roughly what a galaxy is, turns out I didn’t really know about galaxy superclusters.

So in this post I’ll be talking about galaxies, clusters of galaxies, and I’ll explain what the Milky Way has to do with all of this.

In the previous post in this series I talked about solar systems, these are stars (like our Sun) and everything that orbits the star, including planets, asteroids, comets, space stations and so on. Galaxies are next in the size-scale of cosmic objects; galaxies are collections of solar systems.

Often when you look up at the night sky the stars look reasonably well spread out, but this isn’t actually the case. Stars tend to clump together in space, bound into huge clouds of stars by gravity (each star’s mass creates a gravitational attraction to the other stars in the cloud), and this cloud of stars is a galaxy.

A galaxy, basically a cloud of stars

So you can think of Space as being lots of empty, erm, space, with the occasional galaxy of stars. You can think of it like a map of your country, in which most of the houses and people are found grouped together in cities, with less people living in the open countryside. You could even think of yourself as a planet, your house as a solar system, and your city as a galaxy, whereas the countryside is open space. (If you live in the countryside then think of yourself as a wandering star, you lucky romantic).

Think of the cities as galaxies of houses

Galaxies are usually pretty enormous, our own galaxy is thought to contain between 200 and 400 billion stars and if you travelled at half the speed of light (which is very fast) it would take you around 200,000 years to travel from one end to the other (which is very long). Galaxies come in a range of shapes and sizes though. Dwarf galaxies contain only a few billion stars and are thought to be the most common galaxies in our Universe, whilst larger galaxies tend to be either spiral or ellipse galaxies. There are also irregular shaped galaxies too, my favourite of which are ring galaxies.

A ring galaxy (called Hoag’s Object, weirdly)

Galaxies aren’t just made of stars and their solar systems though. They also contain clouds of dust (small fragments of rock) and gas (usually hydrogen and some helium), and black holes. (For a quick reminder, a black hole is an object that has so much mass that it warps space and time into a deep gravity well that sucks in almost everything it encounters).

It’s thought that most galaxies have supermassive black holes at their centre (the biggest type of black hole you can get), which can be many billions of times more massive than our Sun. These supermassive black holes probably provide the enormous gravity needed for the heart of a galaxy (usually called its ‘core’ or ‘active galactic nucleus’) to be held together.

A supermassive black hole (artists impression)

Oh, galaxies are also thought to contain something called dark matter too, lots and lots of it.

Galaxies behave in a confusing way, they spin and move in a fashion that suggests they have more mass than astronomers can detect. When astronomers add up all the visible mass, like all the stars and black holes, as much as 90% of the mass appears to be missing. A bit like your scales telling you you weighed 2,000 pounds (907 kg in real numbers), when your pretty sure you aren’t that big, and should weigh closer to 200 pounds.

Most astronomers think that galaxies contain a special kind of matter, called dark matter, that accounts for this missing mass. Normal matter, often called baryonic matter, interacts with light so it can be seen, or otherwise detected by things like radio telescopes. It’s thought that dark matter doesn’t interact with light though, so it can’t be seen with your eyes, or any of the other scientific tools of humanity. The only way we can detect dark matter is by ‘seeing’ its gravitational affect. It’s thought that 83% of the Universe is composed of this weird dark matter, and it probably acts as the structural foundation holding most galaxies together.

A map of dark matter created from Hubble gravity observations, pretty f*cking cool

I like to think of stars, and other baryonic matter, as the icing that covers the dark matter galaxy cake.

Hhhhmmm dark matter cake

On a side note, many science fiction authors have created stories in which the Universe is full of alien life forms composed of dark matter, such as Stephen Baxter’s Photino Birds. Who knows? If there’s more dark matter than baryonic matter in the Universe, maybe forms of dark matter life have evolved.

So what does the Milky Way have to do with all of this? Put simply, the Milky Way is our Galaxy. It’s a spiral galaxy, with four main spiral arms, it’s got a supermassive black hole at its core, and is thought to be composed of roughly 80% dark matter. Lots of people don’t realise the Milky Way is our Galaxy, probably because you can see it in the sky at night (if you have no clouds and not much light pollution). It looks like a bright white band full of stars, with some black splotches, hence the name, Milky Way.

Looking into the heart of the Milky Way from Earth

But if you can see if from Earth, how can the Earth be part of it? Well its because what you see in the night sky is mostly the centre of the Milky Way. The Earth is found about half-way along one of the spiral arms, so if you’re looking at the right part of the sky, you’re looking into the centre of our own Galaxy.

You are here

Where I live you can never see the Milky Way at night as there’s too much light pollution, but a few times I’ve been lucky to be in a desert, or one time on a small island in Indonesia with no electricity, and the Milky Way forms a truly beautiful band in the night, filled with billions and billions of stars.

Up until the early 1920’s, scientists (and everyone else) thought that the Milky Way was the entire Universe. They thought that one galaxy was all there was. However, the astronomer Edwin Hubble made a momentous discovery in 1923 when he realized that some of the stars he was observing where much further away than he’d originally though, and that they must belong to an entirely different galaxy. It must have been a mind-blowing moment when Hubble realized the Universe was much bigger than we’d thought, that there wasn’t just one galaxy, that there were billions and billions of them.

Today, astronomers have discovered that galaxies themselves tend to clump together. The smallest of these are called galaxy groups, and usually contain around 50 galaxies. Lager groups are called galaxy clusters, and can be massive, containing many thousands of galaxies. These clusters can also be part of still larger structures called galactic superclusters, which contain tens of thousands of galaxies. Just think, for a long time we thought that the Milky Way was the Universe, know we know that a galaxy is a very small component of the Universe indeed, that galaxies are only tiny parts of much larger cosmic structures.

A map of galactic superclusters & voids in space

So there you have it. Galaxies are vast clouds of solar systems held together by dark matter and supermassive black holes. The Milky Way is our galaxy, and it’s part of the Local Group of galaxies, which itself is part of the Virgo Supercluster.

Next, what is the Universe? (This may take some time, as it turns out the Universe is a pretty weird place).

Ive got to hand it to ya.
Brilliant. I’ve been trying to get answers to all these questions for years ( I’m 40, so that’s a few years) and everyone describes things so complicated that it’s just impossible to understand but you simplify everything so beautifully with humour and never sound condescending. As Einstein says “If you can’t explain it simply, you don’t understand it well enough”
I bought myself my first telescope on Monday and am just fascinated of what I’m seeing, especially how quick things move out of focus because of earths rotation. ( I really need to learn how to polar align it)
I have so many questions and you really truly have answered them for me in a language I can understand. So a billion thanks for that.
You should make tv documentaries. Seriously.
Keep it up.
Grainne

Your ability to make this understandable in the kindest and funniest way is amazing! I am also a 40 something year old and have never fully been able to visualize the solar system, galaxy, or universe. Thank you so much for comparing them to concepts that we can understand. You have not only helped me, but now I can translate the topic to my 3rd grade students. Great picture choices, too!